What is endosymbiosis?

The endosymbiosis theory attempts to explain the origins
of organelles such as mitochondria and chloroplasts in eukaryotic cells. The theory
proposes that chloroplasts and mitochondria evolved from certain types of bacteria that
eukaryotic cells engulfed through endophagocytosis. These cells and the bacteria trapped
inside them entered a symbiotic relationship, a close association between different
types of organisms over an extended time. However, more specifically, the relationship
was endosymbiotic, meaning that one of the organisms (the bacteria) lived within the
other (the eukaryotic cells).

According to endosymbiosis
theory, an anaerobic cell probably ingested an aerobic bacterium but failed to digest
it. The aerobic bacterium flourished within the cell because the cell's cytoplasm was
abundant in half-digested food molecules. The bacterium digested these molecules with
oxygen and gained great amounts of energy. Because the bacterium had so much energy, it
probably leaked some of it as Adenosine triphosphate into the cell's cytoplasm. This
benefited the anaerobic cell because it enabled it to digest food aerobically.
Eventually, the aerobic bacterium could no longer live independently from the cell, and
it therefore became a mitochondrion.

The origin of the
chloroplast is very similar to that of the mitochondrion. A cell must have captured a
photosynthetic cyanobacterium and failed to digest it. The cyanobacterium thrived in the
cell and eventually evolved into the first chloroplast. Other eukaryotic organelles may
have also evolved through endosymbiosis; it has been proposed that cilia, flagella,
centrioles, and microtubules may have originated from a symbiosis between a Spirochaete
bacterium and an early eukaryotic cell, but this is not widely accepted among
biologists.

There are several examples of evidence that
support endosymbiosis theory. Mitochondria and chloroplasts contain their own small
supply of DNA, which may be remnants of the genome the organelles had when they were
independent aerobic bacteria.